Charter schools, publicly funded schools that operate autonomously outside the direct control of local school districts and generally enroll students through the choices of their parents rather than through residential assignment, have proven to be popular in Philadelphia. This report examines the effect charter schools are having on student achievement in Philadelphia. In addition to the schools' effect on reading and mathematics achievement, it looks at such questions as what types of students charter schools attract and whether charter schools have higher student turnover rates than traditional public schools. The authors find that students' average gains when attending charter schools are statistically indistinguishable from the gains they experienced while at traditional public schools. Charter schools are attracting students whose prior achievement levels (when they were in traditional district schools) are slightly below the district-wide average, but higher than the average achievement levels of the traditional public schools they left. There is no evidence that the district schools located in neighborhoods with the greatest charter competition are performing any differently as a result of the competition with charter schools.